The present invention relates to a braking system for use principally on a large vehicle and, more particularly, to an eddy-current type retarder for assisting a friction brake.
Eddy-current retarders for braking systems typically include a pair of transversely adjacent support rings each having a number of permanent magnets facing the inner peripheral surface of a brake drum. The polarities of the magnets on each support ring are alternately different in a circumstantial direction and are secured to a fixed frame disposed internally of the brake drum. One of the support rings is fixed to the frame while the other support ring is rotatable to provide relative angular movement between the magnets and thereby switch between an operational mode in which torque is applied to the brake drum and an operational mode in which torque is not applied thereto.
The present applicant has previously disclosed an eddy-current type vehicle braking system in Japanese Patent Application No. 112026/1990 and as shown in FIG. 8. In this eddy-current retarder, a boss portion 5 of a brake drum 7 and a parking brake drum 3 are coupled by a bolt 4 to a flange 2 coupled to an output rotational shaft 1 of a vehicle transmission. The boss portion 5 is joined by a number of radially extending spokes 6 to the right end of the brake drum 7 which is provided on its outer peripheral surface with cooling fins 8. An annular box shaped, fixed frame 22 is formed of a non-magnetic material so as to provide magnetic shielding. The frame 22 defines an annular chamber arranged interiorly of the brake drum 7 which is formed of an electric conductive material.
The fixed frame 22 is secured, for example, to the outer wall of a gear box of the transmission (not shown). First and second magnet support rings 13 and 14, respectively, are formed of magnetic material and are encased in the annular chamber defined by the fixed frame 22. The second support ring 14 is secured to a right end wall of the fixed frame 22 by a bolt 21 while the first support ring 13 is rotatably supported on an inner peripheral wall 9 of the fixed frame 22 by bearings 12. Mounted on the first support ring 13 are a plurality of first permanent magnets 13a circumferentially spaced apart at a given pitch. Similarly, mounted on the second support ring 14 are a plurality of second permanent magnets 14a also circumferentially spaced apart at the given pitch. The permanent magnets 13a and 14a are arranged so that the polarities thereof are alternately different in a circumferential direction.
Provided in an outer peripheral wall 11 of the fixed frame 22 are slots aligned with outer surfaces of the second permanent magnets 14a. A ferromagnetic plate member 15 is fitted into each slot in the outer wall 11. Preferably, the outer peripheral wall 11 is coated with a low-friction film, and the ferromagnetic plates 15 are embedded in the slots in the fixed frame 22.
A hydraulic adjustment actuator 20 formed integrally with a left end wall of the fixed frame 22 functions to rotate the first support ring 13 so as to produce angular movement thereof relative to the second support ring 14. Included in the hydraulic actuator 20 is a piston 17 fitted into a cylinder 18 extending peripherally of the fixed frame 22. An arm 16 protruding from the first support ring 13 through a slot in the left end wall of the fixed frame 22 is connected to a rod extending from the piston 17 outwardly of the cylinder 18.
When braking of the vehicle is not required, the first permanent magnets 13a on the first support ring 13 are positioned such that its poles facing the brake drum 7 are transversely aligned with opposite poles of the second permanent magnets 14a are shown in FIG. 9. Consequently, there are formed magnetic closed circuits each composed of a N-pole of a permanent magnet 14a, a ferromagnetic plate 15, a permanent magnet 13a, a pair of the support rings 13 and 14, and a S-pole of a permanent magnet 14a as shown by the arrows in FIG. 8. The magnetic field directed radially outwardly between the permanent magnets 13a and 14a and the inner peripheral surface of the brake drum 7 therefore is so weak that brake torque is not applied.
When braking action is required, the first support ring 13 is rotated by the hydraulic actuator 20 to position the first and second permanent magnets 13a and 14a such that like poles facing the brake drum 7 are aligned transversely as shown in FIG. 10. Accordingly, as shown in FIG. 11, there are formed closed magnetic circuits each composed of a N-pole of a permanent magnet 14a, a ferromagnetic plate 15, the brake drum 7, an adjacent ferromagnetic plate 15, an adjacent permanent magnet 14a, the support ring 14 and an S-pole of the original permanent magnet 14a. Similar magnetic circuits are formed by the permanent magnets 13a. When the brake drum 7 rotates across the magnetic field extending radially outwardly from the permanent magnets 13a and 14a to the inner peripheral surface of the brake drum 7, an eddy-current is generated which produces a braking torque.
In the aforementioned type of eddy-current retarder, it is preferable that the ferromagnetic plate members 15 be thick to reduce magnetic resistance and thereby reduce the magnetic flux leaked to the brake drum 7 during non-braking periods. However, when the ferromagnetic plates 15 are made thick, the weight of the retarder undesirably increases; the magnetic gap between the permanent magnets 13a, 14a and the inner peripheral surface of the brake drum 7 becomes large to reduce the magnetic field applied to the brake drum 7 during braking; and the dimensions of the permanent magnets 13a, 14a and support rings 13, 14 encased within the fixed frame 22 are restricted.
The object of the present invention, therefore, is to provide an improved eddy-current type braking system in which the above described disadvantages are alleviated.